The present invention provides an open/close valve which contributes the stabilization of the operation responsibility. A housing has a water-in port and a water-out port. Furthermore, a piston which is slidably mounted in the housing in an axial direction by means of an external operation of a shaft, and a pressure offset means for offsetting the pressure which the piston receives from water in the water-out port to the other side are provided. The piston is attached to a seat face of the housing by being slided on one side in an axial direction to close the communication between the water-in port and the water-out port, and the piston is detached to the seat face of the housing by being slided on the other side in an axial direction to open the communication between the water-in port and the water-out port. The pressure offset means comprises a pressure offset room which is formed at the other side of the piston, and a passage for communicating one side of the piston with the pressure offset room.